Synopsis

Description

The /usr/bin/sh utility is a command programming language that executes commands read
from a terminal or a file.

The /usr/xpg4/bin/sh utility is a standards compliant shell. This utility provides all
the functionality of ksh(1), except in cases discussed in ksh(1) where differences in
behavior exist.

The jsh utility is an interface to the shell that provides all
of the functionality of sh and enables job control (see Job Control section
below).

Arguments to the shell are listed in the Invocation section below.

Definitions

A blank is a tab or a space. A name is a
sequence of ASCII letters, digits, or underscores, beginning with a letter or
an underscore. A parameter is a name, a digit, or any of
the characters *, @, #, ?, -, $, and !.

Usage

Commands

A simple-command is a sequence of non-blank words separated by blanks. The
first word specifies the name of the command to be executed. Except
as specified below, the remaining words are passed as arguments to the
invoked command. The command name is passed as argument 0 (see exec(2)).
The value of a simple-command is its exit status if it terminates normally,
or (octal) 200+status if it terminates abnormally. See signal.h(3HEAD) for a list
of status values.

A pipeline is a sequence of one or more commands separated by
|. The standard output of each command but the last is connected
by a pipe(2) to the standard input of the next command. Each
command is run as a separate process. The shell waits for the
last command to terminate. The exit status of a pipeline is the exit
status of the last command in the pipeline.

A list is a sequence of one or more pipelines separated by
;, &, &&, or | |, and optionally terminated by ; or &. Of
these four symbols, ; and & have equal precedence, which is lower
than that of && and | |. The symbols && and | | also
have equal precedence. A semicolon (;) causes sequential execution of the preceding pipeline,
that is, the shell waits for the pipeline to finish before executing
any commands following the semicolon. An ampersand (&) causes asynchronous execution of the
preceding pipeline, that is, the shell does not wait for that pipeline
to finish. The symbol && ( | |) causes the list following it to
be executed only if the preceding pipeline returns a zero (non-zero) exit status.
An arbitrary number of newlines may appear in a list, instead of
semicolons, to delimit commands.

A command is either a simple-command or one of the following. Unless
otherwise stated, the value returned by a command is that of the
last simple-command executed in the command.

forname [ inword . . . ] dolistdone

Each time a for command is executed, name is set to the next word taken from the inword list. If inword . . . is omitted, then the for command executes the dolist once for each positional parameter that is set (see Parameter Substitution section below). Execution ends when there are no more words in the list.

casewordin [ pattern [ | pattern ] )list; ; ] . . . esac

A case command executes the list associated with the first pattern that matches word. The form of the patterns is the same as that used for file-name generation (see File Name Generation section), except that a slash, a leading dot, or a dot immediately following a slash need not be matched explicitly.

iflist; thenlist [ eliflist; thenlist; ] . . .
[ elselist; ] fi

The list following if is executed and, if it returns a zero
exit status, the list following the first then is executed. Otherwise, the
list following elif is executed and, if its value is zero, the list
following the next then is executed. Failing that, the elselist is
executed. If no elselist or thenlist is executed, then the
if command returns a zero exit status.

whilelistdolistdone

A while command repeatedly executes the whilelist and, if the exit status of the last command in the list is zero, executes the dolist; otherwise the loop terminates. If no commands in the dolist are executed, then the while command returns a zero exit status; until may be used in place of while to negate the loop termination test.

(list)

Execute list in a sub-shell.

{list;}

list is executed in the current (that is, parent) shell. The { must be followed by a space.

name( ){list;}

Define a function which is referenced by name. The body of the function is the list of commands between { and }. The { must be followed by a space. Execution of functions is described below (see Execution section). The { and } are unnecessary if the body of the function is a command as defined above, under Commands.

The following words are only recognized as the first word of a
command and when not quoted:

if then else elif fi case esac for while until do done { }

Comments Lines

A word beginning with # causes that word and all the following
characters up to a newline to be ignored.

Command Substitution

The shell reads commands from the string between two grave accents (``)
and the standard output from these commands may be used as all
or part of a word. Trailing newlines from the standard output are
removed.

No interpretation is done on the string before the string is read,
except to remove backslashes (\) used to escape other characters. Backslashes may
be used to escape a grave accent (`) or another backslash (\)
and are removed before the command string is read. Escaping grave accents allows
nested command substitution. If the command substitution lies within a pair of
double quotes (" . . . ` . . . ` . . . "), a backslash used to escape a double quote (\") will
be removed; otherwise, it will be left intact.

If a backslash is used to escape a newline character (\newline), both
the backslash and the newline are removed (see the later section on
Quoting). In addition, backslashes used to escape dollar signs (\$) are removed.
Since no parameter substitution is done on the command string before it is
read, inserting a backslash to escape a dollar sign has no effect.
Backslashes that precede characters other than \, `, ", newline, and $
are left intact when the command string is read.

Parameter Substitution

The character $ is used to introduce substitutable parameters. There are two
types of parameters, positional and keyword. If parameter is a digit, it
is a positional parameter. Positional parameters may be assigned values by set. Keyword
parameters (also known as variables) may be assigned values by writing:

name=value [ name=value ] . . .

Pattern-matching is not performed on value. There cannot be a function and
a variable with the same name.

${parameter}

The value, if any, of the parameter is substituted. The braces are required only when parameter is followed by a letter, digit, or underscore that is not to be interpreted as part of its name. If parameter is * or @, all the positional parameters, starting with $1, are substituted (separated by spaces). Parameter $0 is set from argument zero when the shell is invoked.

${parameter:-word}

Use Default Values. If parameter is unset or null, the expansion of word is substituted; otherwise, the value of parameter is substituted.

${parameter:=word}

Assign Default Values. If parameter is unset or null, the expansion of word is assigned to parameter. In all cases, the final value of parameter is substituted. Only variables, not positional parameters or special parameters, can be assigned in this way.

${parameter:?word}

If parameter is set and is non-null, substitute its value; otherwise, print word and exit from the shell. If word is omitted, the message “parameter null or not set” is printed.

${parameter:+word}

If parameter is set and is non-null, substitute word; otherwise substitute nothing.

In the above, word is not evaluated unless it is to be
used as the substituted string, so that, in the following example, pwd
is executed only if d is not set or is null:

echo ${d:-`pwd`}

If the colon (:) is omitted from the above expressions, the shell
only checks whether parameter is set or not.

The following parameters are automatically set by the shell.

#

The number of positional parameters in decimal.

-

Flags supplied to the shell on invocation or by the set command.

?

The decimal value returned by the last synchronously executed command.

$

The process number of this shell.

!

The process number of the last background command invoked.

The following parameters are used by the shell. The parameters in this
section are also referred to as environment variables.

HOME

The default argument (home directory) for the cd command, set to the user's login directory by login(1) from the password file (see passwd(4)).

PATH

The search path for commands (see Execution section below).

CDPATH

The search path for the cd command.

MAIL

If this parameter is set to the name of a mail file and the MAILPATH parameter is not set, the shell informs the user of the arrival of mail in the specified file.

MAILCHECK

This parameter specifies how often (in seconds) the shell will check for the arrival of mail in the files specified by the MAILPATH or MAIL parameters. The default value is 600 seconds (10 minutes). If set to 0, the shell will check before each prompt.

MAILPATH

A colon-separated list of file names. If this parameter is set, the shell informs the user of the arrival of mail in any of the specified files. Each file name can be followed by % and a message that will be printed when the modification time changes. The default message is, you have mail.

If this parameter is set to the name of a file writable by the user, the shell will write an accounting record in the file for each shell procedure executed.

SHELL

When the shell is invoked, it scans the environment (see Environment section below) for this name.

See environ(5) for descriptions of the following environment variables that affect the
execution of sh: LC_CTYPE and LC_MESSAGES.

The shell gives default values to PATH, PS1, PS2, MAILCHECK, and IFS.
Default values for HOME and MAIL are set by login(1).

Blank Interpretation

After parameter and command substitution, the results of substitution are scanned for
internal field separator characters (those found in IFS) and split into distinct
arguments where such characters are found. Explicit null arguments ("" or '') are
retained. Implicit null arguments (those resulting from parameters that have no values)
are removed.

Input/Output Redirection

A command's input and output may be redirected using a special notation
interpreted by the shell. The following may appear anywhere in a simple-command
or may precede or follow a command and are not passed on
as arguments to the invoked command. Note: Parameter and command substitution occurs before
word or digit is used.

<word

Use file word as standard input (file descriptor 0).

>word

Use file word as standard output (file descriptor 1). If the file does not exist, it is created; otherwise, it is truncated to zero length.

>>word

Use file word as standard output. If the file exists, output is appended to it by first seeking to the EOF. Otherwise, the file is created.

< >word

Open file word for reading and writing as standard input.

<<[-]word

After parameter and command substitution is done on word, the shell input is read up to the first line that literally matches the resulting word, or to an EOF. If, however, the hyphen (-) is appended to <<:

leading tabs are stripped from word before the shell input is read (but after parameter and command substitution is done on word);

leading tabs are stripped from the shell input as it is read and before each line is compared with word; and

shell input is read up to the first line that literally matches the resulting word, or to an EOF.

If any character of word is quoted (see Quoting section later), no additional processing is done to the shell input. If no characters of word are quoted:

parameter and command substitution occurs;

(escaped) \newlines are removed; and

\ must be used to quote the characters \, $, and `.

The resulting document becomes the standard input.

<&digit

Use the file associated with file descriptor digit as standard input. Similarly for the standard output using >&digit.

<&-

The standard input is closed. Similarly for the standard output using >&-.

If any of the above is preceded by a digit, the file
descriptor which will be associated with the file is that specified by
the digit (instead of the default 0 or 1). For example:

... 2>&1

associates file descriptor 2 with the file currently associated with file descriptor
1.

The order in which redirections are specified is significant. The shell evaluates
redirections left-to-right. For example:

... 1>xxx 2>&1

first associates file descriptor 1 with file xxx. It associates file descriptor
2 with the file associated with file descriptor 1 (that is, xxx).
If the order of redirections were reversed, file descriptor 2 would be
associated with the terminal (assuming file descriptor 1 had been) and file descriptor
1 would be associated with file xxx.

Using the terminology introduced on the first page, under Commands, if a
command is composed of several simple commands, redirection will be evaluated for the
entire command before it is evaluated for each simple command. That is, the shell
evaluates redirection for the entire list, then each pipeline within the list,
then each command within each pipeline, then each list within each command.

If a command is followed by &, the default standard input for
the command is the empty file, /dev/null. Otherwise, the environment for the
execution of a command contains the file descriptors of the invoking shell
as modified by input/output specifications.

File Name Generation

Before a command is executed, each command word is scanned for the
characters *, ?, and [. If one of these characters appears the
word is regarded as a pattern. The word is replaced with alphabetically sorted
file names that match the pattern. If no file name is found
that matches the pattern, the word is left unchanged. The character .
at the start of a file name or immediately following a /,
as well as the character / itself, must be matched explicitly.

*

Matches any string, including the null string.

?

Matches any single character.

[. . .]

Matches any one of the enclosed characters. A pair of characters separated by - matches any character lexically between the pair, inclusive. If the first character following the opening [ is a !, any character not enclosed is matched.

Notice that all quoted characters (see below) must be matched explicitly in
a filename.

Quoting

The following characters have a special meaning to the shell and cause
termination of a word unless quoted:

; & ( ) | ^ < > newline space tab

A character may be quoted (that is, made to stand for itself)
by preceding it with a backslash (\) or inserting it between a
pair of quote marks ( ' ' or ""). During processing, the shell may quote
certain characters to prevent them from taking on a special meaning. Backslashes
used to quote a single character are removed from the word before
the command is executed. The pair \newline is removed from a word
before command and parameter substitution.

All characters enclosed between a pair of single quote marks ( ' '), except
a single quote, are quoted by the shell. Backslash has no special
meaning inside a pair of single quotes. A single quote may be
quoted inside a pair of double quote marks (for example, " '"), but
a single quote can not be quoted inside a pair of single
quotes.

Inside a pair of double quote marks (""), parameter and command substitution
occurs and the shell quotes the results to avoid blank interpretation and
file name generation. If $* is within a pair of double quotes,
the positional parameters are substituted and quoted, separated by quoted spaces ("$1 $2 . . .").
However, if $@ is within a pair of double quotes, the positional
parameters are substituted and quoted, separated by unquoted spaces ("$1" "$2" . . .
). \ quotes the characters \, `, , (comma), and $. The
pair \newline is removed before parameter and command substitution. If a backslash precedes
characters other than \, `, , (comma), $, and newline, then the
backslash itself is quoted by the shell.

Prompting

When used interactively, the shell prompts with the value of PS1 before
reading a command. If at any time a newline is typed and
further input is needed to complete a command, the secondary prompt (that
is, the value of PS2) is issued.

Environment

The environment (see environ(5)) is a list of name-value pairs that is
passed to an executed program in the same way as a normal argument
list. The shell interacts with the environment in several ways. On invocation,
the shell scans the environment and creates a parameter for each name
found, giving it the corresponding value. If the user modifies the value
of any of these parameters or creates new parameters, none of these affects
the environment unless the export command is used to bind the shell's
parameter to the environment (see also set-a). A parameter may be
removed from the environment with the unset command. The environment seen by any
executed command is thus composed of any unmodified name-value pairs originally inherited
by the shell, minus any pairs removed by unset, plus any modifications
or additions, all of which must be noted in export commands.

The environment for any simple-command may be augmented by prefixing it with
one or more assignments to parameters. Thus:

TERM=450 command

and

(export TERM; TERM=450; command

are equivalent as far as the execution of command is concerned if
command is not a Special Command. If command is a Special Command,
then

TERM=450 command

will modify the TERM variable in the current shell.

If the -k flag is set, all keyword arguments are placed in
the environment, even if they occur after the command name. The following
example first prints a=b c and c:

echo a=b c
a=b c
set -k
echo a=b c
c

Signals

The INTERRUPT and QUIT signals for an invoked command are ignored if
the command is followed by &. Otherwise, signals have the values inherited
by the shell from its parent, with the exception of signal 11
(but see also the trap command below).

Execution

Each time a command is executed, the command substitution, parameter substitution, blank
interpretation, input/output redirection, and filename generation listed above are carried out. If
the command name matches the name of a defined function, the function
is executed in the shell process (note how this differs from the execution
of shell script files, which require a sub-shell for invocation). If the
command name does not match the name of a defined function, but
matches one of the Special Commands listed below, it is executed in the
shell process.

The positional parameters $1, $2, . . . are set to the arguments of
the function. If the command name matches neither a Special Command nor the
name of a defined function, a new process is created and an
attempt is made to execute the command via exec(2).

The shell parameter PATH defines the search path for the directory containing
the command. Alternative directory names are separated by a colon (:). The
default path is /usr/bin. The current directory is specified by a null
path name, which can appear immediately after the equal sign, between two colon
delimiters anywhere in the path list, or at the end of the
path list. If the command name contains a / the search path
is not used. Otherwise, each directory in the path is searched for
an executable file. If the file has execute permission but is not an
a.out file, it is assumed to be a file containing shell commands.
A sub-shell is spawned to read it. A parenthesized command is also
executed in a sub-shell.

The location in the search path where a command was found is
remembered by the shell (to help avoid unnecessary execs later). If the
command was found in a relative directory, its location must be re-determined
whenever the current directory changes. The shell forgets all remembered locations whenever the
PATH variable is changed or the hash-r command is executed (see
below).

Special Commands

Input/output redirection is now permitted for these commands. File descriptor 1 is
the default output location. When Job Control is enabled, additional Special Commands are
added to the shell's environment (see Job Control section below).

:

No effect; the command does nothing. A zero exit code is returned.

. filename

Read and execute commands from filename and return. The search path specified by PATH is used to find the directory containing filename.

bg [%jobid . . .]

When Job Control is enabled, the bg command is added to the user's environment to manipulate jobs. Resumes the execution of a stopped job in the background. If %jobid is omitted the current job is assumed. (See Job Control section below for more detail.)

break [ n ]

Exit from the enclosing for or while loop, if any. If n is specified, break n levels.

cd [ argument ]

Change the current directory to argument. The shell parameter HOME is the default argument. The shell parameter CDPATH defines the search path for the directory containing argument. Alternative directory names are separated by a colon (:). The default path is <null> (specifying the current directory). Note: The current directory is specified by a null path name, which can appear immediately after the equal sign or between the colon delimiters anywhere else in the path list. If argument begins with a / the search path is not used. Otherwise, each directory in the path is searched for argument.

chdir [ dir ]

chdir changes the shell's working directory to directory dir. If no argument is given, change to the home directory of the user. If dir is a relative pathname not found in the current directory, check for it in those directories listed in the CDPATH variable. If dir is the name of a shell variable whose value starts with a /, change to the directory named by that value.

continue [ n ]

Resume the next iteration of the enclosing for or while loop. If n is specified, resume at the n-th enclosing loop.

echo [ arguments . . . ]

The words in arguments are written to the shell's standard output, separated by space characters. See echo(1) for fuller usage and description.

eval [ argument . . . ]

The arguments are read as input to the shell and the resulting command(s) executed.

exec [ argument . . . ]

The command specified by the arguments is executed in place of this shell without creating a new process. Input/output arguments may appear and, if no other arguments are given, cause the shell input/output to be modified.

exit [ n ]

Causes the calling shell or shell script to exit with the exit status specified by n. If n is omitted the exit status is that of the last command executed (an EOF will also cause the shell to exit.)

export [ name . . . ]

The given names are marked for automatic export to the environment of subsequently executed commands. If no arguments are given, variable names that have been marked for export during the current shell's execution are listed. (Variable names exported from a parent shell are listed only if they have been exported again during the current shell's execution.) Function names are not exported.

fg [%jobid . . .]

When Job Control is enabled, the fg command is added to the user's environment to manipulate jobs. This command resumes the execution of a stopped job in the foreground and also moves an executing background job into the foreground. If %jobid is omitted, the current job is assumed. (See Job Control section below for more detail.)

getopts

Use in shell scripts to support command syntax standards (see Intro(1)). This command parses positional parameters and checks for legal options. See getoptcvt(1) for usage and description.

hash [ -r ] [ name . . . ]

For each name, the location in the search path of the command specified by name is determined and remembered by the shell. The -r option causes the shell to forget all remembered locations. If no arguments are given, information about remembered commands is presented. Hits is the number of times a command has been invoked by the shell process. Cost is a measure of the work required to locate a command in the search path. If a command is found in a relative directory in the search path, after changing to that directory, the stored location of that command is recalculated. Commands for which this will be done are indicated by an asterisk (*) adjacent to the hits information. Cost will be incremented when the recalculation is done.

jobs [-p|-l] [%jobid ...]

jobs-xcommand [arguments]

Reports all jobs that are stopped or executing in the background. If %jobid is omitted, all jobs that are stopped or running in the background will be reported. (See Job Control section below for more detail.)

kill [ -sig ] %job . . .

kill-l

Sends either the TERM (terminate) signal or the specified signal to the specified jobs or processes. Signals are either given by number or by names (as given in signal.h(3HEAD) stripped of the prefix “SIG” with the exception that SIGCHD is named CHLD). If the signal being sent is TERM (terminate) or HUP (hangup), then the job or process will be sent a CONT (continue) signal if it is stopped. The argument job can be the process id of a process that is not a member of one of the active jobs. See Job Control section below for a description of the format of job. In the second form, kill-l, the signal numbers and names are listed. (See kill(1)).

login [ argument . . . ]

Equivalent to `execloginargument. . . .' See login(1) for usage and description.

newgrp [ argument ]

Equivalent to execnewgrpargument. See newgrp(1) for usage and description.

pwd

Print the current working directory. See pwd(1) for usage and description.

readname . . .

One line is read from the standard input and, using the internal field separator, IFS (normally space or tab), to delimit word boundaries, the first word is assigned to the first name, the second word to the second name, and so forth, with leftover words assigned to the last name. Lines can be continued using \newline. Characters other than newline can be quoted by preceding them with a backslash. These backslashes are removed before words are assigned to names, and no interpretation is done on the character that follows the backslash. The return code is 0, unless an EOF is encountered.

readonly [ name . . . ]

The given names are marked readonly and the values of the these names may not be changed by subsequent assignment. If no arguments are given, a list of all readonly names is printed.

return [ n ]

Causes a function to exit with the return value specified by n. If n is omitted, the return status is that of the last command executed.

set [ -aefhkntuvx [ argument . . . ] ]

-a

Mark variables which are modified or created for export.

-e

Exit immediately if a command exits with a non-zero exit status.

-f

Disable file name generation.

-h

Locate and remember function commands as functions are defined (function commands are normally located when the function is executed).

-k

All keyword arguments are placed in the environment for a command, not just those that precede the command name.

-n

Read commands but do not execute them.

-t

Exit after reading and executing one command.

-u

Treat unset variables as an error when substituting.

-v

Print shell input lines as they are read.

-x

Print commands and their arguments as they are executed.

–

Do not change any of the flags; useful in setting $1 to -.

Using + rather than - causes these flags to be turned off. These flags can also be used upon invocation of the shell. The current set of flags may be found in $-. The remaining arguments are positional parameters and are assigned, in order, to $1, $2, . . . If no arguments are given, the values of all names are printed.

shift [ n ]

The positional parameters from $n+1 . . . are renamed $1 . . . . If n is not given, it is assumed to be 1.

Stops the execution of the current shell (but not if it is the login shell).

test

Evaluate conditional expressions. See test(1) for usage and description.

times

Print the accumulated user and system times for processes run from the shell.

trap [ argumentn [ n2 . . . ]]

The command argument is to be read and executed when the shell receives numeric or symbolic signal(s) (n). (Note:argument is scanned once when the trap is set and once when the trap is taken.) Trap commands are executed in order of signal number or corresponding symbolic names. Any attempt to set a trap on a signal that was ignored on entry to the current shell is ineffective. An attempt to trap on signal 11 (memory fault) produces an error. If argument is absent, all trap(s) n are reset to their original values. If argument is the null string, this signal is ignored by the shell and by the commands it invokes. If n is 0, the command argument is executed on exit from the shell. The trap command with no arguments prints a list of commands associated with each signal number.

type [ name . . . ]

For each name, indicate how it would be interpreted if used as a command name.

ulimit [ [-HS] [-a | -cdfnstv] ]

ulimit [ [-HS] [-c | -d | -f | -n | -s | -t | -v] ] limit

ulimit prints or sets hard or soft resource limits. These limits are described in getrlimit(2).

If limit is not present, ulimit prints the specified limits. Any number of limits may be printed at one time. The -a option prints all limits.

If limit is present, ulimit sets the specified limit to limit. The string unlimited requests the largest valid limit. Limits may be set for only one resource at a time. Any user may set a soft limit to any value below the hard limit. Any user may lower a hard limit. Only a super-user may raise a hard limit. (See su(1M).)

The -H option specifies a hard limit. The -S option specifies a soft limit. If neither option is specified, ulimit will set both limits and print the soft limit.

The following options specify the resource whose limits are to be printed or set. If no option is specified, the file size limit is printed or set.

-c

maximum core file size (in 512-byte blocks)

-d

maximum size of data segment or heap (in kbytes)

-f

maximum file size (in 512-byte blocks)

-n

maximum file descriptor plus 1

-s

maximum size of stack segment (in kbytes)

-t

maximum CPU time (in seconds)

-v

maximum size of virtual memory (in kbytes)

Run the sysdef(1M) command to obtain the maximum possible limits for your system. The values reported are in hexadecimal, but can be translated into decimal numbers using the bc(1) utility. See swap(1M).)

As an example of ulimit, to limit the size of a core file dump to 0 Megabytes, type the following:

ulimit -c 0

umask [ nnn ]

The user file-creation mask is set to nnn (see umask(1)). If nnn is omitted, the current value of the mask is printed.

unset [ name . . . ]

For each name, remove the corresponding variable or function value. The variables PATH, PS1, PS2, MAILCHECK, and IFS cannot be unset.

wait [ n ]

Wait for your background process whose process id is n and report its termination status. If n is omitted, all your shell's currently active background processes are waited for and the return code will be zero.

Invocation

If the shell is invoked through exec(2) and the first character of
argument zero is -, commands are initially read from /etc/profile and from
$HOME/.profile, if such files exist. Thereafter, commands are read as described below,
which is also the case when the shell is invoked as /usr/bin/sh.
The flags below are interpreted by the shell on invocation only. Note:
Unless the -c or -s flag is specified, the first argument is assumed
to be the name of a file containing commands, and the remaining
arguments are passed as positional parameters to that command file:

-c string

If the -c flag is present commands are read from string.

-i

If the -i flag is present or if the shell input and output are attached to a terminal, this shell is interactive. In this case, TERMINATE is ignored (so that kill 0 does not kill an interactive shell) and INTERRUPT is caught and ignored (so that wait is interruptible). In all cases, QUIT is ignored by the shell.

-p

If the -p flag is present, the shell will not set the effective user and group IDs to the real user and group IDs.

-r

If the -r flag is present the shell is a restricted shell (see rsh(1M)).

-s

If the -s flag is present or if no arguments remain, commands are read from the standard input. Any remaining arguments specify the positional parameters. Shell output (except for Special Commands) is written to file descriptor 2.

The remaining flags and arguments are described under the set command above.

Job Control (jsh)

When the shell is invoked as jsh, Job Control is enabled in
addition to all of the functionality described previously for sh. Typically, Job
Control is enabled for the interactive shell only. Non-interactive shells typically do
not benefit from the added functionality of Job Control.

With Job Control enabled, every command or pipeline the user enters at
the terminal is called a job. All jobs exist in one of
the following states: foreground, background, or stopped. These terms are defined as
follows:

A job in the foreground has read and write access to the controlling terminal.

A job in the background is denied read access and has conditional write access to the controlling terminal (see stty(1)).

A stopped job is a job that has been placed in a suspended state, usually as a result of a SIGTSTP signal (see signal.h(3HEAD)).

Every job that the shell starts is assigned a positive integer, called
a job number which is tracked by the shell and will be used
as an identifier to indicate a specific job. Additionally, the shell keeps
track of the current and previous jobs. The current job is the most
recent job to be started or restarted. The previous job is the first
non-current job.

The acceptable syntax for a Job Identifier is of the form:

%jobid

where jobid may be specified in any of the following formats:

% or +

For the current job.

-

For the previous job.

?<string>

Specify the job for which the command line uniquely contains string.

n

For job number n.

pref

Where pref is a unique prefix of the command name. For example, if the command ls-lname were running in the background, it could be referred to as %ls. pref cannot contain blanks unless it is quoted.

When Job Control is enabled, the following commands are added to the
user's environment to manipulate jobs:

bg [%jobid . . .]

Resumes the execution of a stopped job in the background. If %jobid is omitted the current job is assumed.

fg [%jobid . . .]

Resumes the execution of a stopped job in the foreground, also moves an executing background job into the foreground. If %jobid is omitted the current job is assumed.

jobs [-p|-l] [%jobid . . .]

jobs-xcommand [arguments]

Reports all jobs that are stopped or executing in the background. If %jobid is omitted, all jobs that are stopped or running in the background will be reported. The following options will modify/enhance the output of jobs:

-l

Report the process group ID and working directory of the jobs.

-p

Report only the process group ID of the jobs.

-x

Replace any jobid found in command or arguments with the corresponding process group ID, and then execute command passing it arguments.

kill [ -signal ] %jobid

Builtin version of kill to provide the functionality of the kill command for processes identified with a jobid.

stop%jobid . . .

Stops the execution of a background job(s).

suspend

Stops the execution of the current shell (but not if it is the login shell).

wait [%jobid . . .]

wait builtin accepts a job identifier. If %jobid is omitted wait behaves as described above under Special Commands.

Large File Behavior

See largefile(5) for the description of the behavior of sh and jsh
when encountering files greater than or equal to 2 Gbyte ( 231 bytes).

Exit Status

Errors detected by the shell, such as syntax errors, cause the shell
to return a non-zero exit status. If the shell is being used
non-interactively execution of the shell file is abandoned. Otherwise, the shell returns
the exit status of the last command executed (see also the exit
command above).

jsh Only

If the shell is invoked as jsh and an attempt is made
to exit the shell while there are stopped jobs, the shell issues
one warning:

There are stopped jobs.

This is the only message. If another exit attempt is made, and
there are still stopped jobs they will be sent a SIGHUP signal
from the kernel and the shell is exited.

If you get the error message, “cannot fork, too many processes”, try using the wait(1) command
to clean up your background processes. If this doesn't help, the system process
table is probably full or you have too many active foreground processes.
There is a limit to the number of process ids associated with
your login, and to the number the system can keep track of.

Only the last process in a pipeline can be waited for.

If a command is executed, and a command with the same name
is installed in a directory in the search path before the directory
where the original command was found, the shell will continue to exec
the original command. Use the hash command to correct this situation.

The Bourne shell has a limitation on the effective UID for a
process. If this UID is less than 100 (and not equal to
the real UID of the process), then the UID is reset to
the real UID of the process.

Because the shell implements both foreground and background jobs in the same
process group, they all receive the same signals, which can lead to
unexpected behavior. It is, therefore, recommended that other job control shells be
used, especially in an interactive environment.

When the shell executes a shell script that attempts to execute a
non-existent command interpreter, the shell returns an erroneous diagnostic message that the
shell script file does not exist.